The role of anti-graft antiserum ("enhancing" serum) in supressing or preventing the development of anti-allograft cell-mediated immunity in inbred mice will be studied in terms of the effects of passive antiserum dosage, biological activities of immunoglobulin classes and subclasses, and the Fc portion of IgG. Passive antiserum dosage parameters will be determined that either augment or depress humoral and cellular immune responses to alloantigens as expressed in graft enhancement or rejection. Studies will be directed toward elucidating the immunocompetent cell populations of mice that cooperate in antibody-mediated suppression of tumor grafts. Methods will be used for separating peritoneal exudate cell population to demarcate and characterize the cells specifically active in antibody-mediated graft rejection.